Sprague-Dawley rats have been studied at six hours and 1, 4, 7, 14 and 21 days following tibial fracture. Biochemical measurements of selected enzymes (alkaline phosphatase, acid phosphatase, pyrophosphatase, beta-galactosidase, and beta-glucuronidase) indicate increased enzymatic activities for all enzymes studied. The onset and extent of these increases are not the same for each enzyme however. Histochemically, at six hours, loss of enzymatic activity is evident in the cells of the fractured bone, and blood vessels in the vicinity of the fracture. From 1-4 days increased periosteal and endosteal cell activity is observed. From 7-14 days after fracture marked increases in all enzymatic activities are noted in callus tissue components as well as blood vessels and perosteum several millimeters proximal and distal to the callus. Distinct differences are demonstrable in the enzymatic activities of "new" invading blood vessels in various callus tissue components. Electron microscopy of cells involved in the fracture healing process have included the identification of unusual vacuolization in osteoblasts 24 hours post fracture, and the observation of a large cellular population within the callus which is intermediate in structure between fibroblasts and osteoblasts or chondroblasts. Effects of dietary alterations on fracture repair are also discussed.